My only thought would be VPU Lockup and Crash. I know I got a set of Drivers that were buggy on a HIS X1900 , where it would freeze and lock the screen until the VPU recovered from the issue.
I Un-install of the Video Drivers, and a Newer Version of Catalyst fixed it..

Just my thoughts at least, considering your Memory Passed the test.

Regards,

ST

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"We know not why he calls for us, only that when he does we must answer" - DMP 2009

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